Musannaf Ibn Abi Shayba 28:735

Ibn ‘Uyaynah narrated to us, from ‘Abdullah ibn Sharik, from Bishr ibn Ghalib, who said: Ibn al-Zubayr asked al-Hasan ibn ‘Ali about a man who fights on behalf of the *Ahl al-Dhimmah* (protected people) and is taken captive. He said: His ransom is from the *kharaj* (tax/tribute) of those people for whom he fought.